Charged-particle-beam (CPB) microlithography apparatus and methods have substantial potential for manufacturing VLSI devices having feature sizes significantly smaller than obtainable with optical microlithography apparatus and methods. Certain CPB apparatus that have been proposed and investigated recently include so-called "spot beam" exposure systems, "variable-shaped-beam" exposure systems, "block" exposure systems, and "divided" projection-transfer systems.
In most CPB microlithography apparatus, an image of a pattern is defined by a mask or reticle (termed generally "reticle" herein). An image of the reticle is projected onto the surface of a wafer or other suitable substrate (termed generally "wafer" herein). In such apparatus, the reticle is normally mounted on a reticle stage and the wafer is mounted on a wafer stage. In other CPB apparatus, the pattern is "written" directly onto the wafer (mounted on a wafer stage) by the charged particle beam without using a reticle at all.
In view of the above, CPB microlithography apparatus can be categorized into two general types. Apparatus of the first type either lack a reticle or, if a reticle is used, the reticle does not move to effect exposure. Rather, the wafer stage continuously moves during the exposure of the pattern onto the wafer. Hence, with apparatus of the first type, the instantaneous exposure location on the wafer is defined by the wafer-stage position and the beam position (as established by one or more deflectors). Apparatus of the second type utilize a reticle and a wafer, and both the reticle and wafer (mounted on respective stages) continuously and synchronously move during exposure of the pattern onto the wafer. Hence, with apparatus of the second type, the instantaneous exposure location on the wafer is defined by the wafer-stage position, the reticle-stage position, and the beam position (as established by one or more deflectors).
In many apparatus of the second type, the wafer stage and reticle stage are synchronously movable in opposite directions along a first dimensional axis (orthogonal to the optical axis) while the charged particle beam is swept along a second dimensional axis (orthogonal to both the optical axis and the first dimensional axis) so as to achieve a scanning exposure of the reticle pattern onto the wafer. During such scanning motion of the stages, the scanning (sweep) velocity of the charged particle beam is usually synchronized to the respective scanning velocities of the stages.
The amount of deflection that can be imparted to the beam, and the speed at which the beam can be deflected, are limited by the ability of the deflector(s) to laterally deflect the beam relative to the optical axis of the apparatus. Hence, every apparatus (whether of the first type or of the second type) has an optimal stage velocity for exposure, and it is possible for a stage (e.g., wafer stage) to have a velocity that is too high for the beam deflection achievable by the apparatus (termed generally a "beam-stage synchronization error"). If the stage velocity is only slightly in excess of the optimal velocity, then beam deflector(s) can usually make corrections needed to ensure accurate pattern transfer. However, if the stage velocity is more than slightly in excess of the optimal velocity, then the beam deflector(s) generally cannot keep up and continuous exposure becomes impossible.
In addition to beam-stage synchronization errors, continuous exposure also can be rendered impossible by (1) excessive synchronization error between the reticle stage and the wafer stage (apparatus of the second type only); (2) a change in beam intensity from what is optimally required for the exposure pattern, especially whenever the scanning velocity of the beam cannot be changed adequately (e.g., slowed) to accommodate such beam-intensity changes; (3) a deviation in the normal synchrononous relationship between stage velocity and exposure due to an irregularity in exposure time for a particular subfield of the reticle; (4) an excessive variation in surface elevation of the wafer and/or reticle; and (5) excessive yaw exhibited by the wafer stage and/or reticle stage.
In conventional CPB microlithography methods and apparatus, further exposure of the die being exposed on the wafer, or even of the remaining dies on the wafer, would be aborted at the moment a condition as listed above occurred. Further exposure could commence upon placing the next die (or the next wafer) into position for exposure. Unfortunately, with such conventional systems, the occurrence of such a problem can result in fatal defects in at least the subject chip, and even in the subject wafer, resulting in marked decreases in device yield per wafer.